1. Field of the Invention
This invention relates to the field of partially or fully implantable hearing aids comprising a transducer which provides direct mechanical excitation of the middle or inner ear. More specifically, this invention relates to such transducers including a housing which can be fixed at the implantation site and a coupling element which can move with respect to the housing, the housing accommodating a piezoelectric element by which the coupling element can transmit vibrations from the piezoelectric element to the middle ear ossicle or directly to the inner ear.
2. Description of Related Art
A transducer of this general type is illustrated in U.S. Pat. No. 5,277,694. In this patent, it is proposed that one wall of the housing be made as a vibrating membrane with an electromechanically active heteromorphic composite element with a piezoelectric ceramic disk attached to the side of the membrane inside the housing. Generally good results have been obtained with a hearing aid transducer built in this manner. However, it has been found that at low frequencies, the coupling element driven by the piezoelectric ceramic disk does not create sufficient deflections to provide adequate loudness level for patients with medium and more serious hearing loss. This insufficient deflection has been attributed, in part, to be caused by the low electrical voltages required for such implants.
U.S. Pat. No. 5,624,376 discloses a transducer for partially or fully implantable hearing aids based on the electromagnetic principle in which a permanent magnet is permanently joined to hermetic housing. An induction coil which interacts with the magnet is permanently joined to the housing wall which is made as a vibratory membrane. On the side of the vibratory membrane outside the housing, the vibratory membrane is provided with a clip element which attaches the transducer to the incus. As AC voltage is applied to the induction coil, the magnet within the housing is displaced thereby causing vibrational excitation of the incus.
The disadvantage of hearing aids provided with these electromagnetic transducers is that the transducer deflection at high frequencies can be too small to achieve a sufficient loudness level for the user. It has been found that in such electromagnetic systems, the electrical impedance increases simultaneously at higher frequencies because of the inductive component. Therefore, broadband electromagnetic systems, for example, those which allow transmission up to 10 kHz, have a high power consumption when compared to piezoelectric systems.
Therefore, there exists an unfulfilled need for partially or fully implantable hearing aids comprising transducers which provide direct mechanical excitation of the middle or inner ear at a sufficient loudness levels at a wide range of frequencies. There also exists an unfulfilled need for such hearing aids which use relatively little amount energy.